The present invention relates to a stent according to the preamble of the independent claim(s) of the present application and a method for the production of such a stent. Such a stent is disclosed for example in U.S. Pat. No. 6,428,569 (Brown).
Stents that are formed from a tubular lattice structure are used for widening and supporting blood vessels. Stenoses or aneurysms in particular are treated in this manner. Stenoses, i.e., narrowing of blood vessels, lead downstream to a restricted supply of the tissue with nutrients. The constriction is opened-up and stabilized using a stent such that an adequate blood flow is guaranteed. Aneurysms represent bulges in blood vessels that lead to severe stress on the blood vessel walls. These bulges may grow with time and cause rupturing of the vessel wall. By placing an appropriate stent in a blood vessel in the region of an aneurysm, the flow of blood in the aneurysm is reduced or stopped such that coagulation of the blood takes place within the aneurysm which prevents it from growing further
During and after implantation, a stent exerts a radial force on the adjacent blood vessel wall. This force is distributed over the number of webs around the stent's circumference. With a relatively small number of webs, each of these exerts a high force on the blood vessel wall, which is undesirable since this may give rise to irritation of or injury to the blood vessel. Due to the resulting inflammatory response, there is an increased risk of restenosis, i.e., renewed narrowing of the blood vessel, after it has been widened by means of a stent. Therefore, in post-published German patent application no. 10 2007 019 772 referring back to the applicant, it is proposed to increase the number of webs such that the forces acting on the vessel wall are better distributed and thus each individual web exerts a lesser force on the vessel wall.
U.S. Pat. No. 6,428,569 (Brown) referred to at the outset describes a stent with a fine-mesh lattice structure, the webs of which have a very small width. The small contact surface between the individual web and the vessel wall arising as a result of the small web width leads to the local pressure, which is exerted on the vessel wall at a specific radial force per web, being relatively high.
The width and number of the webs cannot, however, be increased at will without impairing the stent's implantability. In order to implant the stent, it is compressed and placed inside a catheter which is used to introduce the stent into the blood vessel. The maximum web width or the maximum number of webs is limited by the geometric constraints that are set by the size of the catheter or the size of the blood vessels to be treated.
Added to this is the fact that the force is not transmitted evenly over the entire outer surface of the webs because the webs cut into the vessel wall due to the high local pressure. As a result, the radial force transmitted to the vessel wall is greater in the region of the longitudinal edges of the webs than in the inner region of the webs. The stress concentration thus resulting in the region of the longitudinal edges increases the risk of restenosis.
This effect, which increases with greater web width, is illustrated, for example, in FIG. 1a of the present application, which shows a partial cross-section through a stent according to the prior art in the implanted state. Webs s are in contact with a vessel wall g and transmit a radial force F onto vessel wall g that is brought about due to the stent's expansion. In FIG. 1a, radial force F is indicated by arrows whereby the magnitude of the force is expressed in each case by the length of the arrow. Due to the high local pressure which webs s transmit onto vessel wall g, this wall is stretched and deformed whereby the maximum deformation occurs in the region of longitudinal edges a of webs s. This leads to the transmission of force occurring mainly in the marginal regions or on longitudinal edges a of webs s.
Overall there are, therefore, two structural measures which lead to the required result of reducing the loads on a blood vessel wall by means of a stent. On one hand, an increased number of webs over the cross-section of the stent improves the homogeneous distribution of forces over the vessel wall such that the local forces per web are reduced. On the other hand, a reduction of the local pressures that act on the vessel wall in the region of the individual webs is achieved by increasing the web width and thus the contact area. The combination of both measures, i.e. increasing the number of webs while simultaneously enlarging the web width, is limited by the geometric restrictions during implantation.